Vacuum tube circuit



March 19, 1935. D. D. ROBERTSON VACUUM TUBE CIRCUIT Filed Nov. 11, 1935 COPLANAR KILOCYCLES KILOCYCLES INVENTOR 0.0. ROBERTSON 3/44 A 7' TORNE Y Patented Mar. is, was

um'rso STATES VACUUM TUBE CIRCUIT Donald D Robertson, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, l7. Y., a corporation of New York Application November 11, 1933, Serial No. 697,549

13 Claims.

This invention relates to wave translation and especially to retroaction or feedback in wave translating systems, as for example, in electric wave amplifying systems.

Objects of the invention are to control amplitude and phase of waves in such systems, and it is especially an object of the invention to so control feedback in such systems as to reduce singing tendency in the systems, as for example, to reduce singing tendency in amplifiers that feed back modulation or distortion components for reducing distortion, or feed back fundamental components for increasing stability, or both.

Such amplifiers are disclosed, for example, in a copending application of H. S. Black, Serial No. 606,871, filed April 22, 1932, for Wave translation systems, in H. Nyquist Patent 1,915,440, June 27, 1933, and in British Patents 317,005 and 371,887.

Certain terms and symbols used herein have the following significance. Singing refers to operation such that an impressed small disturbance which itself dies out results in a response that does not die out but goes on indefinitely, either staying at a relatively small value or increasing until it is limited by the non-linearity of the system. The amplification of a vacuum tube amplifier without feedback is designated ,a and is the complex quantity by which the voltage on the grid of the first tube must be multipled to obtain the phase and magnitude of the total resulting voltage in the plate circuit of the last tube. Amplification ratio is the absolute or scalar value of 11. Gain is twenty times the logarithm of the amplification ratio. The quantity p represents the propagation once around the closed feedback loop of a feedback amplifier. It follows that 18 designates the complex quantity by which a driving voltage in the space path of the last tube, in series with the plate-filament impedance in that tube, must be multiplied to give the voltage that it--the driving voltage alone-acting through the feedback path, will produce on the grid of the first tube. As shown in the above mentioned copending application 606,871, the amplification of afeedback amplifier is #B and the corresponding change in amplification caused by the feedback action is -afi The quantity 1 u3 is a quantitative measure of the amount of feedback, and herein, as in that application, the feed.-

back isdescribed as positive feedback or negative feedback according as the absolute value of 1 -H is greater or less than unity.

'In one specific aspect the invention is embodied in vacuum tube amplifiers of the general type in which waves, including those of the range of transmitted frequencies, are so fed back from the output to the input as to reduce the gain of the amplifier below the value that it would have without feedback in order to reduce unwanted modu lation or non-linear effects and render the gain stability greater than itwould be without feedback.

That type of amplifier is disclosed, for example, in the above mentioned copending application and in the above mentioned patents.

In such amplifiers, where tube modulation reduction for modulation components of given frequencies is to be large, it is proportional to the gain (for those modulation components) in a single trip around the closed feedback loop and consequently that gain should be large. The modulation components that it is desired to reduce by feedback are usually waves of frequencies within the utilized frequency range, e. g. within the range of the frequencies of the signal waves to be amplified by the amplifier. In practice, when the loop gain (i. e., the decibel gain for a single trip around the loop) is large for the frequencies of the utilized frequency range, it is greater than zero for some higher frequency and if the loop phase shift (i. e. the phase shift experienced by waves in passing once around the loop) is zero or a multiple of 360 for any frequency at which the loop gain equals or exceeds zero decible, the amplifier may sing at that frequency. (As indicated in the above mentioned copending .application 606,871, a criterion for freedom from singing is given by Nyquists rule, in his article on "Regeneration Theory", Bell System Technical Journal, January, 1932, pages 126 to 147. Such criterion is also given in the above mentioned Nyquist Patent 1,915,440.) Moreover, passive networks introduced in the loop to contribute a component of loop attenuation varying with frequency ordinarily introduce a component of loop phase shift that may tend to change the frequency at which the loop phase shift reaches a given multiple of 360. To avoid the singing condition, it is desirable to control the loop phase shift and the loop gain carefully with respect to the entire frequency spectrum. (Such control is desirable also for other reasons, as for example, to prevent loop phase shift from causing increase in the gain of the amplifier, which increase may be undesirable because accompanied by a corresponding increase in the modulation products. This increase in gain may become a limiting factor determining the permissible loop phase shift at high frequencies where it becomes difficult to obtain sufficient feedback. At other frequencies it is usually not a limiting factor. For example, assuming the amplifier does not sing, the amplifier gain change produced by feedback will be within 1 decibel of the gain around the feedback loop regardless of phase shift'if the gain around the loop exceeds 20 decibels. Fur ther, the feedback does not increase the amplifier gain if the loop gain exceeds 6 decibels.) If the value of the loop phase shift were maintained at 1- 180 it would be as remote as possible from the potential singing values of 0 and multiples of 360; however, in practice it is not necessary to attain this condition. The requirement for freedom from singing will always be met if for every frequency of loop gain, (i. e., every frequency at which the loop gain is zero or greater) the loop phase shift differs from zero and every multiple of 360, or in other words, if the loop phase shift frequency characteristic does not cross nor touch the zero phase shift axis in the frequency range of loop gain. (It is not to be inferred that this requirement is always essential for freedom from singing. A criterion for such freedom is given by Nyquist's rule. as referred to above.)

- In designing an amplifier with negative feedback for distortion reduction, assuming the vacuum tube or tubes of each stage in the loop to introduce a phase shift having a constant component of 180 (in addition to any component due to interelectrode capacitance, for example), the number of vacuum tube stages used in the loop may be made either odd or even, to facilitate control of singing tendency. The question whether an odd or an even number is more suitable will depend upon whether the loop is made to have phase reversing means other than the tubes, and upon what other phase shifts are present in the loop. If, over the frequency range of loop gain the constant component of the total loop phase shift is any odd multiple of 180", then it is necessary that the total variation of the loop phase shift with frequency over that frequency range be maintained within limits of +180 and -180 in order to make the total loop phase shift differ from zero and every multiple of 360 for every frequency in that frequency range.

The difficulty of insuring that the variation of loop phase shift with frequency is maintained within the required limits over the frequency range of loop gain is, in general, increased by the fact that, (as brought out, for example, in the above mentioned copending application), when the distortion reduction and associated amplifier gain reduction produced by feedback action is to be large, the gain of the amplifier without feedback must then correspondingly exceed the gain required with feedback; because when the gain without feedback, required to produce the desired amount of distortion reducing feedback and the desired amount of gain with feedback, necessitates use of a plurality of stages and a plurality of interstage coupling circuits, the phase shifts around the closed loop may become large. For example, they may become large at frequencies well above the utilized range because of shunt capacitance, for instance, tube and wiring capacities. The singing tendency may become particularly troublesome when the amplifier is called upon to transmit wide frequency bands extending to very high frequencies. for example.

I In its specific aspect mentioned above, the invention is a negative feedback amplifier including a vacuum tube having in its cathode lead, common to its control grid circuit and its plate circuit, a frequency selective network, for example, an inductance, a. capacity and a resistance connected in parallel and resonant at a frequency lying above the utilized frequency range and in the neighborhood of a given frequency of loop gain at which the loop phase shift without the network present is zero, the network impedance being low for utilized frequencies and high for the given frequency and producing in the feedback loop, as indicated hereinafter, reduction of loop gain at the given frequency and variation of loop phase shift with frequency that reduce singing tendency of the amplifier without unduly affecting operation of the amplifier deleteriously in the utilized frequency range.

As indicated above, loop gain and loop phase shift present important limitations in operation of feedback amplifiers, especially the loop gain and loop phase shift at high frequencies in operation of wide band negative feedback amplifiers for reducing modulation or distortion by feedback action.

An object of the invention is to control such loop gain and loop phase shift.

It is also an object of the invention to so control the loop gain and loop phase shift as to reduce singing tendency or to increase the distortion suppression obtainable in such amplifiers, or both.

Other objects and aspects of the invention will be apparent from the following description and claims.

In the accompanying drawing, Fig. 1 is a circuit diagram of an amplifier embodying the specific aspect of the invention referred to above; and

Figs. 2 and 3 show curves facilitating explanation of the invention.

In Fig. l a negative feedback amplifier amplifies waves received by input transformer T from line or circuit L1 and transmits the amplified waves through output transformer T to line or circuit L2. The amplifier shown by way of example comprises three vacuum tubes 1, 2 and 3 connected in tandem by interstage networks 4 and 5, and has a bridge type equalizer 6 in its output circuit and a feedback lead 7 extending from the equalizer to a resistance 8 in series with the secondary winding of the input transformer.

The amplifier may be of the general type referred to above as disclosed in the above men tioned copending application, Serial No. 606,871, in which, over the range of transmitted frequencies, for example 8 to 56 kilocycles, loop gain is much greater than zero, and in which fundamental and distortion waves of the range of transmitted frequencies are so fed back as to reduce the amplifier gain below its value for operation without feedback, in order to reduce distortion correspondingly and render the gain stability greater than for operation without feedback.

Thus the interstage networks and the equalizer can be, for example, of the type disclosed in Figs.

and 66 of the application 606,871, the equalizer ize or correct to any desired degree for variation with frequency of attenuation or transmission of the circuit employing the amplifier.

By way of example, tubes 1 and 2 are shown as pentodes and tube 3 as a coplanar grid tube. For instance, tubes 1 and 2 can be heater type tubes such as Western Electric Company type 7592-A tubes, and tube 3 can be a Western Electric Company type 281-A tube as described in the above mentioned application 606,871 and in Pidgeon-McNally Patent 1,920,274, August 1, 1933.

The cathode heating circuits for the amplifier tubes are not shown, (as they can readily be supplied by those skilled in the art), except that a winding 9 is shown which may be, for example, the secondary winding of a transformer supplying heating current to the filamentary cathode of tube 3.

R1 and R2 are grid biasing resistors for tubes 1 and 2, respectively, and are by-passed for alternating current by condensers C1 and C2, respectively.

A network N comprising inductance L, resistance R and capacity connected in parallel in the cathode lead of one of the tubes, for example, tube 1, common to the grid and plate circuits of the tube, provides frequency selective amplitude and phase control of waves transmitted by the tube, for example, to reduce tendency of the amplifier to sing around the feedback loop including lead 7 at a frequency above the utilized frequency range, for instance, at a frequency such as fp of Fig. 2 in the neighborhood of 400 kilocycles in the case of the 8 to 56 kilocycle amplifier.

Figs. 2 and 3 show curves typical of amplifiers such as that of Fig. 1. In each of these two figures the abscissae are, frequencies, in kilocycles, and the ordinates are phase angles and gains. In Fig. 2 curves G1 and P1, respectively, show the loop gain and the angle of the loop phase shift (for the amplifier feedback loop including the feedback lead 7) when the network N is omitted (e. g. short-circuited); and in Fig. 3, curves G and P, respectively, show the loop gain and the angle of the loop phase shift for the amplifier with the network N functioning. In Fig. 2, the curves G2 and P2, respectively, show the component of loop gain and the component of loop phase shift that the network N contributes to the amplifier. Thus, curve P of Fig. 3 is obtained by adding curves P1 and P2 of Fig. 2; and curve G of Fig. 3 is obtained by adding curves G1 and G: of Fig. 2. The component of loop gain produced by network N is a negative gain, i. e., a loss. The transmission band, or utilized frequency range of the amplifier is indicated in Fig. 2 as from 8 to 56 kilocycles.

Curve P1 shows the loop phase shift (without N) as a small negative angle (in the fourth quadrant) at a frequency below the utilized frequency range. As the frequency increases from that value to a frequency in the utilized range the curve shows the loop phase shift increasing its negative value until it becomes -180=+180. Then as the frequency continues to increase the curve .shows the loop phase shift decreasing from 180 to angles in the first quadrant. At still higher frequencies the curve shows the loop phase shift approaching zero, which it reaches at frequency iv, a frequency shown as above and in the neighborhood of 400 kilocycles. The frequency f is therefore a potential singing frequency, i. e. a. frequency at which the amplifier without N may sing (around the loop including lead '7) since curve G1 shows the loop gain greater than zero at that frequency of zero loop phase shift. However, the network N prevents the amplifier from singing.

A cathode lead impedance consisting of resistance common to the grid and plate circuits of the tube would tend to reduce the gain for transmission through the tube, without producing phase shift. If a capacity be connected in parallel with the resistance, the cathode lead network, consisting of resistance and capacity in parallel, tends to produce in the transmission through the tube a loss and a phase shift, the loss decreasing with increase of frequency from zero frequency to infinite frequency and the phase shift increasing from zero to a maximum with increase of frequency from zero and then decreasing to zero with increase of frequency to infinite frequency. If the cathode lead network consists of inductance and resistance in parallel, it tends to produce in the transmission through the tube a loss and a phase shift, the loss increasing with increase of frequency from zero loss at zero frequency to maximum loss at infinite frequency, and the phase shift decreasing from zero to a maximum negative value with increase of frequency from zero and then increasing to zero with increase of frequency to infinite frequency. If the cathode network is inductance, capacity and resistance in parallel, as in the case of network N in Fig. 1, it tends to produce in the transmission through the tube a loss and a phase shift, as indicated by curves G2 and P2, respectively, in Fig. 2. The phase shift increases from zero to a maximum negative value with frequency increase from zero, then decreases to zero with further'increase of frequency to the resonant or critical frequency of the network (shown as 400 kilocycles in Fig. 2), then increases to a positive maximum with further increase of frequency, and then decreases to zero with further increase of frequency to infinity. The loss increases with frequency from zero loss at zero frequency to maximum loss at the resonant or critical frequency of the network and then decreases with further increase of frequency to zero loss at infinite frequency.

With the resonant frequency of the network N below and in the neighborhood of the potential singing frequency f as shown in Fig. 2, the net-' work N can be made to contribute to the amplifier loop gain and loop phase shift, such compononts (shown by curves G2 and P2) as to lower the frequency at which the loop gain reaches zero (and changes from a positive value to a negative value or a loss) from the frequency value shown at fa in Fig. 2 to the frequency value shown at fa in Fig. 3, and at the same time to raise the frequency at which the loop phase shift reaches zero (and changes sign) from the frequency value shown at f in Fig. 2 to the frequency value shown at ,f in Fig. 3. Thus, with the network N functioning, as frequency increases, above the utilized frequency range, the loop gain becomes a loss before the loop phase shift reaches zero, and consequently the singing tendency ofthe amplifier is reduced or the singing margin increased, and the loop gain permissible in the utilized frequency range is increased, so increased distortion suppression and amplifier gain and gain stability can be obtained.

The grid cathode voltage produced on the control grid of the tube 1 by network N may depend not only upon that network, but also to an appreciable extent upon the interstage network 4 and the properties of the tube and the other components of the amplifier. However, where it is not desired to utilize this dependence, its effect can be made small or kept within desired limits by suitable design, as will be apparent to those skilled in the art.

What is claimed is:

1. A wave translating system comprising a closed feedback loop, an active transducer included in said loop, means included in said loop for producing negative feedback in said transducer of waves in the utilized frequency range said loop tending to sing at a frequency above the utilized frequency range, and means in said loop for reducing said singing tendency of the loop, said transducer comprising an electric space discharge device having an anode, a cathode, a discharge control element, an input circuit between said control element and said cathode, an output circuit between said anode and said cathode, and a cathode lead common to said input and output circuits, and said last mentioned means comprising a frequency selective impedance in said lead.

2. A wave translating system comprising a closed feedback loop, an amplifier in said loop, means in said loop for producing negative feedback in said amplifier of waves in the utilized frequency range to render modulation in said amplifier less than without feedback, said loop tending to sing at a frequency outside of the utilized frequency range, and frequency selective means in said loop for reducing the singing tendency of the loop, said amplifier comprising a vacuum tube having an anode, a cathode, a grid, an input circuit connecting said grid and said cathode, an output circuit connecting said anode and said cathode, and a cathode lead common to said input and output circuits, and said frequency selective means comprising inductance, capacity and resistance in parallel in said lead.

3. A wave translating system comprising a closed feedback loop, an amplifier included in said loop, means included in said loop for producing negative feedback in said amplifier in the utilized frequency range, said loop tending to sing at a frequency above the utilized frequency range, and means in said loop for reducing the singing tendency of the loop, said amplifier comprising a vacuum tube having an anode, a cathode, a grid, an input circuit between said grid and said cathode, an output circuit between said anode and said cathode, and a cathode lead common to said input and output circuits, and said last mentioned means comprising an impedance in said lead, of value high for said frequency and low for the utilized frequencies, to reduce the loop gain at said frequency.

4. A wave translating system comprising a closed feedback loop, an amplifier included in said loop, means included in said loop for producing negative feedback in said amplifier in the utilized frequency range, said loop tending to sing at a frequency above the utilized frequency range, said amplifier comprising an electric space discharge device having an anode, a cathode, a grid, an input circuit between said grid and said cathode, an output circuit between said anode and said cathode, and a cathode lead common to said input and output circuits, and a frequency selective impedance in said lead producing in said loop a component of loop phase shift that reduces said singing tendency.

5. A wave translating system comprising a closed feedback loop, an amplifier in said loop, means in said loop for producing negative feedback in said amplifier of waves in the utilized fre quency range, said amplifier comprising an electric space discharge device having an anode, a cathode, a grid, an input circuit between said grid and said cathode, an output circuit between said anode and said cathode, and a cathode lead common to said input and output circuits, and means in said lead for increasing the minimum departure of the loop phase shift from zero and multiples of 360 over the frequency range of loop gain lying above said utilized range.

6. A wave translating system comprising a closed feedback loop, an amplifier in said loop, means in said loop for producing negative feedback in said amplifier of waves in the utilized frequency range, said amplifier comprising an electric space discharge device having an anode, a cathode, a grid, an input circuit between said grid and said cathode, an output circuit between said anode and said cathode, and a cathode lead common to said input and output circuits, and inductance, capacity and resistance in parallel in said lead, resonant at a frequency above said utilized range, for increasing the minimum departure of the loop phase shift from zero and multiples of 360 over the frequency range of loop gain lying above said utilized range.

7. An amplifier having a feedback path forming therewith a loop producing negative feedback therein in the operating frequency range of the amplifier but producing tendency of the amplifier to sing around said loop at a frequency at least substantially as high as the upper limit of said range, said amplifier comprising an electric space discharge device having an anode, a cathode, a discharge control element, an input circuit between said control element and said cathode, an output circuit between said'anode and said cathode, a cathode lead common to said input circuit and said output circuit, and means for reducing said singing tendency, said means comprising an impedance in said lead, producing in said loop a component of loop phase shift at said frequency substantial in magnitude and of the same sign as the phase shift of the remainder of the loop at neighboring lower frequencies.

8. An amplifier having a feedback path forming therewith a loop producing negative feedback therein, said loop having its phase shift change sign at a frequency within the frequency range of loop gain and at least substantially as high as the upper limit of an operating frequency band for the amplifier, said amplifier comprising an electric space discharge device having an anode, a cathode, a discharge control element, an in-- put circuit between said control element and said cathode, an output circuit between said anode and said cathode, a cathode lead common to said input circuit and said output circuit, and an impedance in said lead for reducing tendency of the amplifier to sing around the loop, said impedance introducing in the loop a. component of loop phase shift that raises the frequency value at which the loop phase shift executes said change of sign.

9. An amplifier having a feedback path forming therewith a loop producing negative feedback therein, said loop having its phase shift change sign at a frequency within the frequency range of loop gain and at least substantially as high as the upper limit of an operating frequency band for the amplifier, said amplifier comprising an electric space discharge device having an anode, a cathode, a discharge control element, an input circuit between said control element and said cathode, an output circuit between said anode and said cathode, a cathode lead common to said input circuit and said output circuit, an

impedance in said lead for reducing tendency of l ing therewith a loop producing negative feed-v back therein, said loop having its phase shift zero at a frequency which is in the range of loop gain and which is at least substantially as high as the upper limit of an operating frequency range of the amplifier, said amplifier comprising an electric space discharge device having an anode, a cathode, a discharge control element,

an input circuit between said control element and said cathode, an output circuit between said anode andisaid cathode, a cathode lead common to said input circuit and said output circuit and impedance comprising inductance and capacity in parallel in said lead and resonant in the neighborhood of said frequency for producing in said loop a reduction of loop gain at said frequency to reduce tendency of the amplifier to sing around said loop at said frequency. 11. An amplifier having a feedback path form ing therewith a loop producing negative feedback therein, said loop having its phase shift change sign in a given direction with frequency increase at a frequency within the frequency range of the loop gain and at least substantially as high as the upper limit of an operating frequency band for the amplifier, said amplifier comprising an electric space discharge device having an anode, a cathode, a discharge control element, an input circuit between said control element and said cathode, an output circuit between said anode and said cathode, a cathode lead common to said input circuit and said output circuit, and an impedance in said lead for reducing tendency of the amplifier to sing around the loop, said impedance introducing in the loop a component of loop phase shift that changes sign in a direction opposite to said given direction with frequency increase at a frequency below and in the neighborhood of said first mentioned frequency and raises the frequency value at which the loop phase shift changes sign, and saidimpedance lowering the upper limit of the frequency range of loop gain below said raised frequency value.

12. An amplifier having a feedback path forming therewith a loop producing negative feedback therein, said loop having its phase shift zero at a frequency which is in the range of loop gain and which is at least substantially as high as the upper limit of an operating frequency range of the amplifier, said amplifier comprising an electric space discharge device having an anode, a cathode, a discharge control element, an input circuit between said control element and said cathode, an output circuit between said anode and said cathode, a cathode lead common to said input circuit and said output circuit, and means for reducing tendency of the amplifier to sing around said loop at said frequency, comprising an impedance in said lead introducing in said loop a component of loop phase shift that changes the loop phase shift for said frequency from zero to a finite value.

13. An amplifier having afeedback path forming therewith a loop producing negative feedback therein, said loop having its phase shift zero at a given frequency, which is in the range of loop ain and which is at least substantially as high as the upper limit of the operating frequency range of the amplifier, said amplifier comprising an electric space discharge device having an anode, a cathode, a discharge control element, an input circuit between said control element and said cathode, an output circuit between said anode and said cathode, a cathode lead common to said input circuit and said output circuit, and an impedance in said lead introducing in said loop a component of loop phase shift which is zero at a frequency below and in a the neighborhood of said given frequency and DONALD n. ROBERTSON. 

